When a resource is released (freed) it should be de-accounted
from the resource counter it was accounted to. This is called
- "uncharging".
+ "uncharging". The return value of this function indicate the amount
+ of charges still present in the counter.

> It is useful to know how many charges are still left after a call to
> res_counter_uncharge. While it is possible to issue a res_counter_read
> after uncharge, this can be racy.
>
> If we need, for instance, to take some action when the counters drop
> down to 0, only one of the callers should see it. This is the same
> semantics as the atomic variables in the kernel.
>
> Since the current return value is void, we don't need to worry about
> anything breaking due to this change: nobody relied on that, and only
> users appearing from now on will be checking this value.
>
> Signed-off-by: Glauber Costa <glommer@parallels.com>
> Reviewed-by: Michal Hocko <mhocko@suse.cz>
> Acked-by: Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> CC: Johannes Weiner <hannes@cmpxchg.org>
> CC: Suleiman Souhlal <suleiman@google.com>
> CC: Tejun Heo <tj@kernel.org>